Method of making junction transistors



May 27, 1958 R. L. LONGINI 2,836,520

METHOD OF MAKING JUNCTION TRANSISTORS Filed Aug. 17. 1953 7 17 f7 2; .fij.

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INVENTOR.

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- lrrakxvey United States Patent METHOD OF MAKENG JUNCTION TRANSISTORS Richard L. Longini, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 17, 1953, Serial No. 374,547

11 Claims. (Cl. 148-15) This invention relates to a method of making junction transistors of the kind in which one type of donor or acceptor material is caused to alloy with a part of a body of a semiconductor material. This is done by melting a pellet of the first type of material which is a doping impurity, in contact with a larger body of the semiconductor material.

A particular feature of the invention resides in the fact that after the alloy has been formed the assembly is subjected to gradient cooling during which the molten constituent of the solid body in the molten pellet is caused to redeposit on the solid body, thereby extending in a regular and even manner the crystal lattice of the solid body. This particular type of redeposition is favorable for the reliable production of fused junctions having desirable characteristics.

The gradient cooling is preferably carried out by causing the large body of the second type of material to cool faster than the molten alloy, thereby causing some of the second type of material to solidify out of the alloy and become redeposited on the body of second type material.

The invention also contemplates means for forming a smooth interface between the pellet of the first type of material and the body of the second type of material.

Figure 1 is a perspective schematic view of a fused junction transistor;

Fig. 2 is a view in elevation of a wafer of germanium with a pellet of indium placed thereon;

Fig. 3 is a View in elevation of the assembly of Figure 2 disposed on a graphite block and with a graphite block placed above it;

Fig. 4 is a vertical cross-section through a fused junction unit;

Fig. 5 is a view in elevation of the unit of Figure 4 being processed further; and

Fig. 6 is a view in elevation of a complete fused junctiontransistor.

The operation of a transistor is dependent upon certain materials called semiconductors, such as germanium and silicon. When one of these semiconductors contains a minute quantity of a donor impurity, such as antimony, it is called N-type material, and when it contains a minute quantity of an acceptor impurity, such as indium, it is called P-type material. A boundary face between these two materials is called a P-N junction.

A junction transistor is one in which an electrical current passing through the transistor must pass through two of these P-N junctions, arranged back-to-ba-ck. Thus, in one type of junction transistor, a body of N- type material has disposed on each of two opposite sides a body of P-type material. The electrical current is caused to pass first through one of the P-type bodies; through the P-N junction; through the N-type body; through the N-P junction; and then through the other P-type body.

Fig. 1 illustrates diagrammatically a junction transistor ice in which ill indicates a body of P-type material, 11 indicates a body of N-type semiconductor material, md 12 indicates another body of P-type material. A lead 13 is connected to the body It), which is called the emitter, a lead 14 is connected to the body 11, which is called the base, and a lead 15 is connected to the body 12, which is called the collector. These junction transistors have been made by using for the body 11 a thin wafer of N-type germanium material and alloying bodies 10 and 12 of P-type material, such as indium, to opposite sides of the wafer 11.

The junction transistor described above was known at the time of the present invention, and is mentioned herein merely for reference purposes.

The melting point of indium (156 C.) is much lower than the melting point of germanium (958 C.), and when the above assembly is heated to cause alloying, germanium is dissolved in the liquid indium. Thus, at 609 (3., the molten alloy contains 22% germanium. As the temperature is lowered some of the germanium crystallizes out of the molten alloy, and is redeposited on the original body 11 of germanium, thus extending the molecular lattice of the body of germanium.

I have found that improved results may be obtained by controlling the cooling of each of the junctions between the germanium N-type material and the molten indium in such a manner that there is a temperature gradient with the liquid metal maintained at a higher temperature than the solid metal. Since this gradient is in opposite directions on the two sides of the relatively thin body 11 of germanium, the process is preferably conducted in separate steps as indicated in the following illustrative example.

A relativelythin piece of N-type germanium, indicated diagrammatically at 16 in Fig. 2, is placed in a suitable furnace and a pellet 17 of indium is placed on top of the piece of germanium. The assembly is then heated to a temperature in the range 525 to 600 C., which can be called temperature A. While this is the preferred temperature range, it should be understood that some variation from this range is permissible. As a specific example we will select the temperature of 600 C., at which temperature the indium is molten, and forms an alloy. with the germanium, with 22% of germanium dissolved in the indium.

2. First gradient cooling The assembly is now subjectedv to a drop in tempera-- ture, which may be of the order of 15 to 30 (3., (temperature drop B), under conditions which insure that the main body of germanium is cooled faster than the alloy. In other words, a temperature gradient is maintained with the liquid alloy kept hotter than the solid germanh :n. This cooling step may last from 5 to 20 minutes.

Any suitable apparatus can be used to carry out this gradient cooling. As illustrated diagrammatically in 3 the pellet 17 is now molten, and is in the form of a verb ton of alloy on the body 16 of germanium. The body 16 rests on a block of graphite 1S, and a larger cap 19 of graphite is held above the pellet 17 in order to retard its cooling. It will he understood that the pieces of graphite 18 and 19 were in place during the first heating step, so that they were heated throughout to the temperature of 606 C.

Carrying forward the specific example, the assembly is now subjected to a drop of 30 C., or to 570 C. During this cooling step, the larger cap 1% will retain its heat longer than the smaller block 18, and hence the piece of germanium 16 will cool faster and be at a lower tempera-" ture than the button 17 of alloy. Obviously the steepness of the temperature gradient can be controlled by selecting graphite pieces of different relative sizes. a

The desired gradient cooling can of course be carried out by other means, as by using appropriate heating or} cooling means.

At most, the temperature gradient is relatively small, and exists over' a relatively short distance. Hence the temperature of 570 C. would be a mean of the temperatures of the liquid and the solid.

During this first gradient cooling, germanium solidifies out of the cooling alloy 17 and is redeposited on the solid germanium body, as indicated diagrammatically in Fig. 4.

This figure indicat'es that the molten pellet 17 of alloy rests in a slight pan-like or basin-like depression caused by melting of the germanium into the alloy pellet. During the gradient cooling, due to thefact that the body aesaeao r of germanium is cooler thanthe pellet 17 of alloy,

germanium solidifies out of the alloy and is redeposited on the surface of the pan-like depression as indicated as a layer 29 on an exaggerated scale in Fig. 4. This rede- V posited germanium contains a small amount of indium, which can diffuse into the main body 16 of germanium.

In the specific example given'above, the alloy at 600 C. contained 22% of dissolved germanium, and at 570 C. contains 18% of. dissolved germanium. Therefore z of the originally dissolved germanium ,is still in solution at 570 C., and of the originally dissolved germanium is redeposited. This redeposited germanium the germanium. From a molecular standpoint this rede- 5. Second gradient cooling The assembly'is now subjected to a second gradient temperature must be sufficient to cause the redeposition of a layerof germanium from" the melt of indium and germanium alloy under the button formed when pellet-22 is melted, after the fashion indicated at 20in Fig. 4. This redeposited germanium layer must be of sufficient thickness that it ,will not be wiped out subsequen'tly by.

the operational fluctuations of furnace heat.

6. Difiusion heating The assembly is now held at the lowered temperature (560 C. inthe specific, illustration) for a period 1mg enough to permit the desired diffusion at the boundaries. in the specific illustration this period would be for about I 20 minutes.

posited germanium may be said to build up or extend the a crystal lattice of the germanium. By extending the lattice in a regular and even manner the regeneration rate of holes and electrons near the interface will 'be kept low, leading to a low value of saturation. current, which 1s a desirable characteristic.

The thickness of the layer of redeposited germanium will of course depend upon the temperatures used, and may in some cases be as thick as 10m1crons.

The temperature drop selected for "this first gradient heating should be sufficiently large that theda'yer of redeposited germanium will not be entirely redissolved during subsequent heating or by average operational fluctua-- tions of the heat of the furnace. Thus, if this temperature drop is too smalL subsequent heating steps, or operational fluctuations of furnace temperature, may bring the temperature of the assembly back up to 600 C. and thus wipe out completely the efiects of the first gradient heating.

3. First natural cooling During this step the assembly is removed from the furnace and is permitted to cool to normal room temperature so that the button 17 is completely sohdified.

4. Second alloy heating The assembly is now turned over and laid on a piece of graphite which has a'surface depression to accommodate the now-solid button 17. 'A pellet 22 of indium, which may be smaller than pellet 17, is now placed on body 16.

The assembly is again placed in the furnace and is heated to temperature C, which is preferably slightly'below the figure obtained by subtracting temperature drop B from temperature A. In the specific illustration temperature C will be approximately 565 C. [temperature A (600 C.) minus temperature drop B (30 0.):570"

Referring to Fig. 4 it should be noted that the layer zil of redeposited germanium is saturated with indium. During'the diffusion heating, indium diffuses from thislayer 20 across the boundary into the main body 16 of ger- 7 manium. The distance to which the indium diffuses d e; pends upon the time and temperature of-the diffusing heating. Thus at 560 C. the indium will, in 1000 seconds, diffuse into the germanium a distance of 4x10 em.

At 525 C. the same penetrationof indium requires ten' 7 times as long a diifusion period.

Some diffusion of the indium into the germanium took place during the periods of gradient cooling, and therefore in some. cases it may beunnecessary to employ a sep arate diffusion heating step.

7. Second natural cooling The assembly is again removed from the furnaceand is permitted to cool to room: temperature so that the molten pellet 22 solidifies.

Resulting transistor as the base, having an attached lead 14. The indium button 17,which resulted from the first gradient cooling, formed P-type material adjacent its boundary with the base, and serves as the .collector. A lead 15 is con nected to the collector. The indium button 22, which resulted from the second gradient cooling, also formed P-type material adjacent its boundary with the base, and serves as theemitter. A lead 13 is connected to the emitter, and a lead 14 is connected to the base Theassembly is given a final treatment in an etching solution to remove surface short circuiting. For the etch a 1:1:4 solution of hydrofluoric acid, nitric acid and water is suitable.

The process of the present invention results in a favorable redeposition of germanium at each of the P-N junctions, but the, different treatment of these two junctions results in different characteristics, the characteristics of each junction being those most desired for that particular junction.

For the collector junction it is desirable to have a minimum number of defects and trapping centers on the impurity side of the collector junction, since the current is carried by holes. Only the back resistance of this junction is critical. The first gradient cooling as described above yields these desirable characteristics,

and gives the collector high resistivity.

I For the emitter junction it is desirable to have a large concentration gradient on the impurity side of the junction, so that a copious supply of carriers will be available. The emitter should have a low forward resistance, and the second gradient cooling as described above re sults in an emitter junction with low forward resistance and other good emitter characteristics.

One of the distinct advantages of this invention is that the extent of indium diffusion can be closely controlled, and since the distance between the two germaniumindium interfaces gives the working thickness of the base, it is possible to have a very thin working base (desirable for good transistor qualities) while using a relatively thick piece of base material (desirable for ease of manufacture) Smooth interface When indium is melted against germanium, often the indium wets the germanium at local spots, and the germanium melts first at these spots, resulting in pits in the germanium. In order to avoid this localized wetting, with the resulting pitting, certain precautions may be taken.

One such precaution is slow heating, that is, bringing the assembly up to the alloying temperature very slowly. This slow heating will tend to cause the indium to wet and dissolve the germanium evenly over the interface. Prior investigators have considered fast heating to be preferable, but I have discovered that slow heating is of distinct advantage. By slow heating I mean bringing the assembly up to 525 C. in from /2 to 2 hours.

Another possible precaution is to apply a thin film of gold to the surface of the germanium block, as by the evaporation process. The gold causes the indium to wet the surface evenly, thus causing an even attack of the molten indium on the germanium.

Still another way to avoid localized wetting, with its consequent pitting, may be called appeasement of the indium. This method involves alloying with the indium enough germanium to saturate the indium at the wetting temperature. The amounts of germanium needed to saturate the indium at certain usable wetting temperatures are as follows:

At 325 C. need 2% germanium At 400 C. need 5% germanium At 500 C. need germanium In the specific example given above, where the first alloy heating is carried to a temperature of 600 C., the appropriate wetting temperature may be taken as 400 C., which means that 5% germanium should be added to the indium. After this alloy is formed it is raised to a temperature of 800 C. and is then chilled rapidly to room temperature, asby oil quenchin The result of this chilling is that the germanium becomes distributed throughout the alloy in the form of small crystals in intimate contact with and surrounded by indium. Small pieces of the indium-germanium alloy thus prepared may be used for the pellets 17 and 22 in the process described above.

The process is now started by placing a pellet 17 of this indium-germaniumalloy on the body 16, and the assembly is subjected to the first alloy heating described above. When the assembly reaches the temperature of 400 C. the small crystals of germanium present in the pellet 17 are first wet by the indium, since they are in favorable size and location to be completely wetted. These small crystals now dissolve in the indium, saturating it with dissolved germanium, and permitting the alloy pellet 17 to uniformly wet an area of the body without dissolving any germanium from the body 16. As the temperature rises above 400 C., the indium-germaniumalloy no w dissolves a thin. and uniform layer form layer of germanium being subsequently 'redeposited in the manner previously explained.

The principle of appeasement has been explained above in connection with the use of germanium as the material of the main body 16, and of indium as the material of the pellet 17. It should be understood, however, that any materials having similar properties may be substituted for germanium and indium, the important consideration being that the material used for the pellet 17 be appeased with the material to be used for the main body 16.

It is important that the interface of a P-N junction be as smooth as possible, because:

(1) Any protuberances on the interface will cause intense local fields which may initiate low back-voltage break-down. Therefore a smooth interface gives greater reliability.

(2) The constancy of the distance between the junctions determines the sharpness of the characteristics of the transistor.

(3) The smoother the interfaces resulting from a particular commercial process, the more uniform will be the characteristics of a resulting group of transistors.

Further modifications The P-N junction formed by steps 1, 2 and 3 above (the collector junction) has uses independently of the subsequent steps of the process. Thus, it may be used as the collector junction of a transistor having the rest of its parts made in the ordinary manner, or some other manner. Also, this junction is well adapted for rectifier use.

In the above description, under step No. 3, first natural cooling, it is stated that the assembly is cooled to room temperature. This is done as a matter of convenience so that the assembly may be turned over and the second pellet added outside of the controlled atmosphere. But with suitable manipulative procedures, these operations can be carried out without removing the assembly from the protective atmosphere, in which case the assembly need be cooled only to some temperature slightly below the freezing temperature of pellet 17. This procedure will conserve heat, and will permit keeping pellet 17 in place while the assembly is turned up-side-down.

It will be understood, of course, that the heating operations described above are carried out in suitable pro tective atmospheres, and that the temperatures and times will be varied according to the different materials being used, but following the general principles outlined above.

Possible materials The N-type material may be produced by using any donor material, such as arsenic, phosphorus or bismuth, or alloys thereof, such as bismuth-antimony alloy. The P-type" material may be formed by using any acceptor material, such as aluminum, gallium, or boron, or alloys thereof.

Also, instead of producing a P-N-P type of junction transistor, the method can be used to produce N-P-N transistors. Thus the body 16 may be formed of P-type germanium, and the pellets 17 and 22 may be formed" of an antimony-germanium alloy, preferably the eutectic alloy or enriched with germanium slightly beyond the eutectic alloy.

' Conclusion racemes: invention produces superior-and more um m l form" junctions, thus resulting'in superior electrical devices and a lowering of costs dueto' iewer rejectsdurifig steps comprising: placing a pellet COIHPrising a doping impurity material selected from the group consisting of donors and acceptors ona body ofla semiconductor material selected from the group consisting of silicon, g n manium and silicon-germanium alloys, to formlan assemhly, heating the assernhlyto afirsttemperature above the meltingpoint of the pellet material but .belowfthe melting point of the semiconductor materialpwhereby the pellet material melts,.dissolves some of thesemiconductor material and forms abutton of amolten alloy ofthetwo materials, and then coolingthe assembly ito a mean temperature ofthe order of 15 Crto 30 C. below said first temperature and maintaining a gradient so that the body of semiconductor materialisat a "lower temperature than the button of. the molten alloy, the assembly being maintained at said mean'temperature which is below the liquidus temperature of ,the .alloy for ,axperiod of time thereby causing the semiconductor materialto redeposit from the molten alloy upon the b'ody of semiconductor material and for the doping material to dilfuse into the semiconductor material. j

2. A method as called for in claim 1 in which the pellets comprises indium and the semiconductor material is germanium.

'3. A method as called for in claim 1 in which thepellet comprises germanium-indium alloys and the semiconductor is germanium. 7

4. -A method as called for in claim 1 in which after the gradient cooling the assembly is subjected to a-diffusion heating during which the assembly is held at a temperature just below the temperature reached at the end of thegradient cooling, therebycausingtheimpurity material to difiuse into the second typematerial.

5. In the method of making junction transistors,- the steps-comprising: placing a pellet comprising an impurity material'on one portion of a body of a semicondncto'r material selected from the group consisting of silicon, germanium and silicon 'gerrnanium alloys, to form an assembly, heating the assembly to a first temperature above the melting point of the pellet material but below the meltiug point of the semiconductor material, whereby the pellet material melts, dissolves some ofthe semiconductor material and forms a'button of molten alloy of the two materials, cooling the assembly to a mean temperature of the order of 15 C. to 30 C. lower than said first tem-' perature and maintaining a gradient so that'thebody of semiconductor material is at a lower temperature than the button of the molten alloy, the assembly being maintained 'at the mean"ten 1perature which is below the liquidus temperature of the alloy for a pe'riod of time thereby causing the semiconductorrnaterial to redeposit from the molten alloy onto thebodyof semiconductor material and for the impurity material to diffuse into the semiconductor materifl, thereby forming a first junction on one side of the body of semiconductor material,. and repe'atingthe stepsofi the process for alloying a second pellet comprising'an impurityimate'ri al on another portion 7 of the bodytof the semiconductormaterial, thei'ebyforrne inga second junction onthebodypf semiconductor mater ial, the temperature during the second alloying process aaeee of the pellets comprise indium and the semiconductor maten-arts ge manium v i m h 7. The method as calledforin' claim 6 in "which both of the pellets' comprise enes semiconductor material isjf erm anium.

8. The method of claim material is silicon;

steps comprising: placing a pellet of an impurity material on a first side of a body of .a semiconductor material terial, wherebythe pellet material melts, dissolves a part of the semiconductor material and forms a first button o'f mo lten alloy of the two materials, .coolingLthe assembly so that the body of semiconductor material teaches and maintains a :first cooling :mean temperature of the order of 15 C. to 3'0' Q lower than theffirst temperature and slightly lower than that of the first button of molten alloy, the 'ass'emblybeing maintained at the mean temperaturewhicliis below theiliq iidus temperature of the alloy 'for'a period of-time thereby causing semiconductor material to redeposit from the moltenalloy onto thefirst side of the body of semiconduetor'material and for the impurity to dilfuse intoth'e semiconductormaterial .there by forminga'fimt junction on the firstside brine body thereafter-rapidly mana es-preliminary assembly tea ductor material and forms a second button ermin alloy'of the two materials," cooling Jthe finalassembly so that the body ofsemiconductor material is at a lower mean temperature of the order of 5 'Chthan the said second temperature and slightly less than the second button of moltenalloy, thereby causing semiconductor material to .redeposit from the. secondtliuttoii of molten alloy upon the second side of the solidbody of semiconductor material, thereby forming a second junction on said opposite side of the body of second type, and thereafter cooling the final assemblytoroom temperature,

10. The method as called for in claim; 9 in which-ithe pellet comprises indium and the semiconductor material is germanium. f

.11. .Themethodas called .forinclaimfidn which the semiconductor. material is. silicon.

References Cited-in the file of this patent UNITED STATES .r Are rs di m 5119i n s; n t an the sent-ta trum 

1. IN THE METHOD OF MAKING JUNCTION TRANSISTORS THE STEPS COMPRISING: PLACING A PELLET COMPRISING A DOPING IMPURITY MATERIAL SELECTED FROM THE GROUP CONSISTING OF DONORS AND ACCEPTORS ON A BODY OF A SEMICONDUCTOR MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICON, GERMANIUM AND SILICON-GERMANIUM ALLOYS, TO FORM AN ASSEMBLY, HEATING THE ASSEMBLY TO A FIRST TEMPERATURE ABOVE THE MELTING POINT OF THE PELLET MATERIAL BUT BELOW THE MELTING POINT OF THE SEMICONDUCTOR MATERIAL, WHEREBY THE PELLET MATERIAL MELTS, DISSOLVES SOME OF THE SEMICONDUCTOR MATERIAL AND FORMS A BUTTON OF A MOLTEN ALLOY OF THE TWO MATERIALS, AND THEN COOLING THE ASSEMBLY TO A MEANS TEMPERATURE OF THE ORDER OF 15*C. TO 30*C. BELOW SAID FIRST TEMPERATURE AND MAINTAINING A GRADIENT SO THAT THE BODY OF SEMICONDUCTOR MATERIAL IS AT A LOWER TEMPERATURE THAN THE BUTTON OF THE MOLTEN ALLOY, THE ASSEMBLY BEING MAINTAINED AT SAID MEANS TEMPERATURE WHICH IS BELOW THE LIQUIDUS TEMPERATURE OF THE ALLOY FOR A PERIOD OF TIME THEREBY CAUSING THE SEMICONDUCTOR MATERIAL TO REDEPOSIT FROM THE MOLTEN ALLOY UPON THE BODY OF SEMICONDUCTOR MATERIAL AND FOR THE DOPING MATERIAL TO DIFFUSE INTO THE SEMICONDUCTOR MATERIAL. 